Seniority and Overtime issues

I'm an LPN at a small facility. I have been working there for a long time and have seniority over most nurses there. I depend on overtime pay to pay the bills and for other needs. Yet it seems I'm on the bottom of the totem pole when it comes to getting overtime hours--they just started hiring a lot of brand new nurses, and my overtime is being taken away and given to them. These nurses get to see the work schedule first and fill it up with the overtime they want, and I'm not allowed to see the schedule and am forced to take what hours might be left. With all the new hires, it's very difficult to grab any overtime.

I was always under the impression that seniority counts in any workplace. Isn't this true? I'm afraid my finances will be jeopardized if i don't work a certain amount of overtime hours. What actions should i take to correct the situation?